Despite that pressure, Romney ignored his rivals and kept his focus on President Barack Obama.
"This is a contest about the economy and about the budget and about foreign affairs, but it's also an election that is bigger than that," Romney said at the Mississippi Valley Fairgrounds, calling the election a battle for the "soul of America."
Santorum, the former Pennsylvania senator, sounded increasingly optimistic.
"I look forward to building on our momentum from Iowa to again defy expectations in New Hampshire," a confident Santorum said Monday, announcing the endorsement of several New Hampshire politicians and making clear his intent to compete aggressively in that state's Jan. 10 primary regardless of the Iowa outcome.
Monday is the last full day of campaigning before caucuses that kick off state-by-state voting in the fight to pick a Republican to challenge Obama in November.
The trio clustered at the top, as well as the trailing Texas Gov. Rick Perry, former House Speaker Newt Gingrich and Minnesota Rep. Michele Bachmann, were fanning out across the Midwestern state to make closing arguments to the chunk of likely Iowa Republican caucus-goers who say they haven't decided who to support and still could change their minds.
"I feel very confident. We've got a great ground game," Perry said on NBC as the day began, highlighting the 41 percent of likely voters who say in a recent poll that they could be persuaded to vote for someone else.
Bachmann, who was born in Waterloo but later moved to Minnesota, rolled out a TV ad - her first since before an August test vote in Iowa - reminding Republican caucus voters of her Iowa roots and stiff stands in Washington. The ad twice mentions Bachmann's Iowa heritage and calls her "one of our own."
It's been a costly race with at least $12.5 million in advertising - much of it negative - flooding Iowa's airwaves in the run-up to the caucuses as candidates and outside groups aligned with them, called super PACs, worked to influence the outcome of what remains a wide open race.
Romney, the former Massachusetts governor, has roughly the same amount of backing in polls as he did in 2008 when he lost the race with 25 percent of the vote amid skepticism over his Mormon faith and reversals on some social issues. This year, he has been counting on the GOP conservative base splintering in a multicandidate field to allow him to win with roughly the same percentage of the vote.
But now, Santorum is challenging him for the lead, and the anti-abortion crusader is looking to unify socially conservative voters behind his candidacy. And that's putting pressure on Romney, who is focusing on turning out his base of support from his last campaign.
Romney aides say they still feel confident heading into Tuesday's contest - but they've become more careful in recent days to qualify their expectations.
"It's a question of whether we'll be better off the day after Iowa than we were the day before," said Stuart Stevens, Romney's top strategist. "Hopefully we will be."
Romney has consistently pressed his central argument that only he has the ability to wage a strong enough campaign to beat Obama. Santorum, popular among cultural and religious conservatives, and Paul, a libertarian-leaning candidate, worked to persuade Iowans that they would be able to attract a broad enough coalition of voters in the general election to beat Obama.
On Sunday, Romney engaged with Santorum for the first time since his rival's rise, offering mild criticism. Santorum "has spent his career in the government in Washington," Romney said during a campaign stop in Atlantic.
"I'll let people make their own assessment of our respective records," Romney said. "But I'm a conservative."
Santorum, in turn, worked to counter Romney's electability argument.
"I'm the candidate that actually was able to win in states, as a conservative, in getting Democrats and independents to vote for us," Santorum, the former Pennsylvania senator who is surging in the race and is a favorite among cultural conservatives, said in an interview on CNN. "Mitt Romney has no track history of doing that."
Paul, too, worked to counter the suggestion that he's a fringe candidate, calling himself "electable" in a Sunday interview with ABC from his home state of Texas. He's looking to stem a recent slide as he's been attacked as outside the mainstream on foreign policy.
The issue of what type of candidate to choose cuts to the heart of why the race is so volatile. An NBC/Marist poll last week showed nearly even percentages of Iowa caucus-goers want a candidate who shares their values as want a candidate who can beat Obama.
And a Des Moines Register poll released Saturday showed half of likely caucus-goers viewed Romney as the Republican most likely to win the November election. On that question, he was far ahead of Santorum and Paul, who was viewed as least likely to win. But Santorum beat Romney best when asked who relates best to Iowans.
Romney's support was steady in the poll with 24 percent, while Paul was sliding and Santorum rising in a race that will come down to which way on-the-fence Republicans here break. All the other candidates trailed.